This invention relates to heating furnaces for metal blooms or billets in which the blooms or billets are aligned transversely to their direction of movement through the furnace, and particularly to such a furnace having an internally cooled member located above the blooms or billets and extending parallel to them adjacent the exit end of the furnace.
In the production of seamless steel pipe, blooms are typically reheated prior to the piercing operation in a furnace of the walking beam type. In these furnaces, 101/2 inch square blooms, for example, separated by air gaps of 8 to 9 inches are conveyed through the various heating zones by a walking beam mechanism which periodically lifts each bloom, advances and then deposits it back on water cooled skid pipe rails. The furnace usually has three heating zones, e.g., preheat, heating and soak zones. Each zone has top and bottom burners for applying heat to the adjacent surfaces of the blooms. To remove cold spots on the bottom of the blooms due to contact with the skid rails, the bottom burners in the heating and/or soak zones may be fired harder than the top burners. However, due to the gap or spacing between the blooms, additional heat radiates to the furnace roof and is reflected back to the top surface of the blooms. Thus, temperature differentials in the vertical direction commonly occur to varying degrees in the blooms. Additionally, horizontal temperature gradients are caused due to the "cold door effect" at the exit end of the furnace. Cold air enters the furnace through the discharge door and causes horizontal temperature variation in the blooms. Also, the fact that the heating zone is generally hotter than the soak zone, causes the same effect of horizontal temperature variation. A possible solution would be to rotate the blooms in the furnace by a turning mechanism. Such a device would necessarily be of complex mechanical design, as well as difficult and uneconomical to operate.
U.S. Pat. No. 2,776,128, Nesbitt, et al., describes a forge furnace having water-cooled skid rails which support elongated metal workpieces as they pass longitudinally through a fast heating section of the furnace. Toward the discharge end of the furnace, the workpieces are passed onto an underfired open-work refractory hearth designed to remove cold spots on the bottom surfaces of the workpieces from the skid rails. At this location, the water-cooled skid rails are extended upwardly above the workpieces to form "shadow" rail sections in a shielding position out of contact with the workpieces between their upper surfaces and the hot furnace wall. By the shielding of the shadow rail sections, the top surfaces of the workpieces which reach the desired discharge temperature faster than the cooler bottom surfaces are prevented from overheating. In this reference, the shadow rails extend parallel to the direction of workpiece flow rather than transversely as in applicant's invention. Applicant's invention permits the effect of an internally cooled member to be concentrated near the furnace exit where it is needed most for minimizing vertical temperature gradients or at some other distance from the exit to minimize horizontal temperature gradients. It also permits correction of temperature differentials in both a vertical and horizontal direction in the workpieces.
U.S. Pat. No. 2,940,741, Nesbitt, discloses a forge furnace for the fast heating of billets. The furnace comprises a refractory lined tunnel forming a cylindrical chamber. The reference states that in a furnace having a circular combustion chamber which is tangentially fired, it is possible to maintain combustion against the refractory wall of the furnace as a relatively thin sleeve in the chamber. It also states that it is possible to maintain unusually high wall temperatures for rapid "high heat head" heating. The temperatures maintained in the heating chamber, generally in excess of 2600.degree. F., are so high as to practically preclude the use of other than refractory or water-cooled work supports in the furnace chamber. The method of firing demands that the water-cooled supports be kept out of the sleeve or path of burning gases in the furnace chamber; otherwise, they would chill the gases before combustion was complete. The reference further states that where the workpieces are required to be pushed through the furnace broadside first, the ends of the workpieces tend to become overheated. This is due to a radiation phenomenon where the furnace wall temperature is considerably in excess of the final desired work temperature. To prevent this overheating, a pair of internally cooled side rails are provided and form a channel between which the work pieces are moved. Where the shadow rails are not used, non-uniform heating of the ends overheats the steel, making a sticky scale which rapidly deteriorates forging dies. The scale is worked into the surface of the workpiece in the dies and is very detrimental to the dies, causing materially shortened die life and subsequent machining troubles. This reference does not address and shows no recognition of the problem of decreasing temperature differentials in a vertical direction in workpieces being transported through a furnace.
Other references of interest are U.S. Pat. Nos. 234,162; 782,697; 1,535,132; 1,841,692; 2,689,119; 2,735,229; and 3,342,468.